Currently, instruments for measuring biomedical signals are very popular medical devices and can be applied in military field, biomedical field, man-machine system field, and so on. Biomedical signal measuring instruments may include electroencephalograph (EEG), electrocardiograph (ECG), etc. The EEG records the electrical activity of neurons within the brain via electrodes placed on the scalp. Since the EEG is a non-invasive instrument and is able to reflect the activity of brain cells within a few milliseconds (ms), it has been widely applied in medical diagnosis and neurobiological research.
Traditionally, the electrode for the EEG is a wet electrode. Before placing the wet electrode on a patient's skin, it is necessary to apply a layer of conductive gel on the patient's skin. The conductive gel might cause discomfort to some patients, such as allergy or swelling. And, the conductive gel tends to become dried with time and therefore has lowered electric conductivity when the measurement has continued for a longer period of time. That is, the wet electrode requiring the application of conductive gel could not be used over a long period of time in measuring the biomedical signal. Further, when the wet electrode is to be placed on an area with densely distributed hair for measuring biomedical signal, the hair must be properly shaved off the skin in advance to ensure that the wet electrode can measure accurate biomedical signal. Moreover, in the conventional way of measuring the brain electric signal, several tens of wet electrodes are needed at the same time. Since not all the wet electrodes have the same electric conductivity, a lot of time will be consumed in necessary pre-process and time adjustment before the measurement can start.
Compared to the wet electrode, the dry electrode applied in biomedical signal measurement is more convenient for use and provides more advantages. However, the current dry electrode is manufactured through microstructure process, such as micro-electro-mechanical system (MEMS), or using carbon nanotubes. Dry electrodes with such structures tend to break in use, and could not be used at body areas with hair. These disadvantages prevent the dry electrode from being widely accepted in the measuring of biomedical signal.